#13264: GHC panic with (->) generalization branch while compiling lens -------------------------------------+------------------------------------- Reporter: bgamari | Owner: Type: bug | Status: new Priority: normal | Milestone: 8.2.1 Component: Compiler | Version: 8.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by goldfire): I see what's going on here. `tcRepSplitTyConApp_maybe` rightly needs to know the `RuntimeRep` of the argument type `([] |> <*>_N ->_N Sym {alzj}) a_alzd`. This `RuntimeRep` is buried in the from-type of the coercion hole `{alzj}`. We can see in the trace that `{alzj}` has from-type `k_agxC`. Except this has already been filled in with `*`. So the problem is that we haven't zonked yet -- and that's it. `can_eq_nc'` works over non-flat (that is, unzonked) types and also over flat (zonked) types. See `Note [Canonicalising equalities]` immediately above. Your decomposition will work on the zonked types. So returning `Nothing`, as you propose, works, as it forces `can_eq_nc'` to zonk and then try again, succeeding. Is `tcRepSplitTyConApp_maybe` used anywhere else? If so, returning `Nothing` in this case may be wrong.... except that returning `Nothing` in this case makes `tcRepSplitTyConApp_maybe` strictly more well-defined, so I suppose nothing can actually go wrong with this change. A detailed `Note` about the choice for `tcRepSplitTyConApp_maybe` not to panic is called for. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13264#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler